Well-logging instruments are used in boreholes to make, for example, formation evaluation measurements and infer properties of the formation surrounding the borehole and the fluids in the formations. Such well-logging instruments may include resistivity logging tools that measure the resistivities of earth formations surrounding a borehole, such as in a hydrocarbon (e.g., oil, natural gas, etc.) well. One approach for performing resistivity measurements is by lowering a wireline-conveyed logging device into a borehole after the borehole is drilled.
Another approach is to make such measurements while the well is being drilled, which is referred to as logging-while-drilling (LWD) or measurement-while-drilling (MWD). LWD or MWD techniques may allow corrective actions to be taken during the drilling processes if desired. For example, borehole information, if available, in real time may be used to make adjustments to mud weights to prevent formation damage and to improve well stability. In addition, real time formation log data may be used to direct a drill bit to the desired direction (i.e., geosteering).
Some well-logging tools have coils or electrical conductors mounted on the housing forming a downhole tubular. These conductors are connected to electronic devices contained inside the housing, which protects the electronics from the harsh downhole environment. A feedthrough connects the exposed conductors to the electronic devices contained within the housing. In order to maintain good electrical conductivity under harsh operating conditions, the conductor is protected by an overmolding in rubber or a thermopolastic or thermoset polymer, e.g., Teflon®. In higher temperature downhole operating conditions above 180° C., however, Teflon® and similar polymers have a reduced lifetime and often stop functioning as a seal altogether, resulting in the loss of insulation and loss of conduction as electrical connections are exposed to the harsh downhole environmental conditions.